Phonon hydrodynamics and ultrahigh-room-temperature thermal conductivity in thin graphite

TL;DR

This study demonstrates that thin graphite exhibits ultrahigh room-temperature thermal conductivity due to phonon hydrodynamics, with conductivity increasing as thickness decreases, highlighting the role of phonon dispersion anisotropy.

Contribution

It reveals the link between high thermal conductivity, thickness, and phonon hydrodynamics in thin graphite, a novel insight into heat transport in carbon allotropes.

Findings

Room temperature thermal conductivity of 8.5 μm graphite is 4300 W/m·K.

Thermal diffusivity increases with temperature, indicating hydrodynamic phonon flow.

Decreasing thickness enhances thermal conductivity, linked to phonon momentum relaxation.

Abstract

Allotropes of carbon, such as diamond and graphene, are among the best conductors of heat. We monitored the evolution of thermal conductivity in thin graphite as a function of temperature and thickness and found an intimate link between high conductivity, thickness, and phonon hydrodynamics. The room temperature in-plane thermal conductivity of 8.5-micrometer-thick graphite was 4300 watts per meter-kelvin-a value well above that for diamond and slightly larger than in isotopically purified graphene. Warming enhances thermal diffusivity across a wide temperature range, supporting partially hydrodynamic phonon flow. The enhancement of thermal conductivity that we observed with decreasing thickness points to a correlation between the out-of-plane momentum of phonons and the fraction of momentum relaxing collisions. We argue that this is due to the extreme phonon dispersion anisotropy in graphite.